8 Claims. '(Cl. 324-40) This invention relates to nondestructive eddy current testing devices and more particularly to multiple parameter nondestructive eddy current testing devices. The invention described herein was made in the course of, or

under, a contract with the US. Atomic Energy Commisspacing, a single test frequency does not produce suflicient' information to determine the three variables unambiguousl Fiirther, even where additional variables are not of direct interest, their effects may mask those of the main variables thereby reducing the effectiveness of the test. Thus, using a single frequency gives a limited amount of information which in some cases is sutficien't but which generally leaves much of the test specimen information unrevealed.

It is therefore .one 'obiect of the present invention to provide means'for nonckstructively measuring multiple parameters of a test specimen.

It is another object of the present invention to provide nondestructive eddy current means for simultaneously measuring multiple parameters of a test specimen.

It is another object of the present invention to provide a nondestructive eddy current device using multiple frequencies for measuring simultaneously multiple parameters of a test specimen.

Other objects of the present invention will become more apparent as the detailed description proceeds.

In general, the present invention comprises a probe coil and means to drive the probe coil with a signal having a broadband of frequencies. Means are then provided for analyzing the signal detected by the probe coil to give individual parameter values for the test specimen.

Further understanding of the present invention will best be obtained from consideration of the accompanying drawings in which:

FIG. 1 is a schematic diagram of the preferred embodiment for the practice of the present invention.

FIG. 2 is a more detailed schematic diagram of the device of FIG. 1.

FIG. 3 is a detailed schematic diagram of the generator and signd expander section of the device of FIGURE 2.

FIG. 4 is an alternate schematic diagram of the analyzer section of the device or FIGURE 1.

In FIGURE 1, a generator supplies a multidimensional excitation current to test coils 12 and balance coils 14. 'The generator 10 also provides a reference signal to analyzer 16. The output from test coils 12 and. balmadam Pmo upon-the application of two basic principles in addition to those used in conventional eddy current nondestructive testing. The first principle is that more information can be carried by a broadband signal than one of a single frequency, and the second is that the broadband output fromthe test coils can be analyzed to give the desired information regarding test specimen parameters It is to be noted that generally a particular parameter will affect each frequency in an applied broadband signal. How

ever, it will afiect each frequency differently. Further, the effect on each frequency between different parameters will generally not be the same.

In operation, the generator 10 supplies a multidimensional excitation current to test coils 12 and balance coil: 14. The generator 10 also supplies a reference or timing signal to the analyzer section 16. The output of the gen eratormay be in the form of single or repetitive pulses, combinations of rising or decaying exponential signals, or combinations of a number of sinusoidal signals. Thus, the excitation signal'frorn generator 10 may be constructed to simplify the tasl: of the analyzer section 16.

As stated, the output of the generator 10 is fed to the test coils 12. These coils are essentially similarin construction and operation to those of conventional eddy current nondestructive testing equipment except thatthey must be operable over the desired broad frequency bandwidth without generating spurious responses. The balcoupling, it is difficult to obtain a null condition with only the balance coils 14. Therefore a trimmer balance unit '26 is used to provide a fine null adjustment. Balance trimmer unit 26 is of conventional design, of the basic type described by Donald L. Waiderlich in Pulsed Eddy Currents Gauge Plating Thickness, Electronics, November 1955, pages 146 and 147. Operation at or near a null signal permits observation of small parameter variations about nominal parameter values.

The null or near null input .to amplifier 18 is amplified thereby before transmission to the analyzer 16. The amplifier 18 is a broadband amplifier having low 'noise and low distortion. In some cases,an automatic gain control circuit may be incorporated therein to reduce signal amplitude variations due to changes in signal caused by variations in coupling between the test coils 12 and test specimen 24.

The output from amplifier 18 is a complex signal which is accepted by analyzer 1610 give in responsethereto a ance coils 14 is fed through amplifier 18 to the input of analyzer 16. The output of analyzer 16 has a plurality of parameter readout meters 20. The operation of the device of FIGURE 1 is based plurality of output signals each of which depends only upon an individual parameter value of the test specimen.

Reference is made to FIGURE 2 to further explain the operation of the device of FIGURE 1 and more particularly the analyzer section thereof.

The output from amplifier 18 is fed to a signal expander 28 wherein the signal is expanded so that the outputs C C32, C and C5 therefrom, called signal descriptors, represent the coefficients of the components of the signal on a selected basis. Thus, it is to be understood that the number of outputs shown herein and described are not limited to four, the number being chosen merely as illustrative for description and as representative iv Patented Jan. 11, 1966 7' the polarity switches 42 and the polarity selected outputs C C C and C3; arethen fed to potentiometers 44. The wiper arm outputs of the potentiometers 44 are fed to summing amplifiers 46 and thence to parameter readout meters 48, each meter thereby indicating a particular parameter under test.

To facilitate explanation of operation, the operation of the signal expander 28 will be described later. It 1s sufiicient for the present description that the. expander 28 expands the complex signal from amplifier 18 using Fourier series expansion. Thus the complex signal input t is expanded on a Fourier series basis so that the descriptor outputs C C C and C represent the various coefficients of the Fourier series expansion of the signal. The signal descriptor outputs C C32, C and C from expander 28 are slowly varying D.-C. signals which are constant for a fixed set of test specimen parameters. As any test parameter changes, all the signal descriptors therefore will generally vary, but in slightly difierent de-" C] =the signal descriptors Q arranged in a column matrix [A] =matrixjrelating the P; to the Q P] =parameter values arranged in a column matrix The values of the A in [A] can be determined experi- C C C and C are then fed to the amplifiers 38 of exampleQGeneral Radio impedance comparaton Type' 1605A). The output of the' generator section 52,

a 100 kc. signal, is fed to a 100 kc. isolation amplifier 56 and also to a decade frequency divider 58;}The

decade frequency divider 58 divides the 100 kc. 'in- I i put signal by ten to give an output signal of 10 kc. The i 10 kc. output signal oi-the frequency divider 58 is fed to v the 10 kc. generator section "60' of a second commercial impedance comparator 62'. The output of the generator section 60, a 10 kc. signal, is fed to the input of a 10 kc. v isolation amplifier 64. The outputs of isolation amplifiers 56 and 64 are-fed to power amplifier 66 and then tol test coils 12 and balance coils 14. The isolation amplifiers 56 and 64 prevent cross modulation between the l0 and 100 kc. generators.

The detected signal from test coils l2 and balance coils 1 4 is transmitted to a balance trimmer 68 to give a'null or near null output for predetermined reference values. The output signal from balance trimmer 68 is transmitted through a preamplifier 70 to a 10 kc. bandpass filter 72 and a 100 kc. bandpass filter 74.

The output of the 10 kc. bandpass filter 72 is fed to the input of the amplitude-phasedetector section- 76 of the 10 kc. impedance comparator -62, and the output of' i the 100 kc. bandpass filter 74 is fed tothe input of the amplitudephase detector section 78 of the 100 kc. impedanoe comparator 54. The outputs C C31, C and C of the amplitude-phase detectors 74 and 78 are slowly varying D.-C. signals which .are constant for a fixed set of test specimen parameters and which represent the Fourier expansion of their input signal. The outputs EIGURE 2.

As previously stated, the impedance comparators 54 and 62 are commercially available and each may be divided into two sections, a generator section and an ammentally by varying the parameters one at a. time and reading the corresponding values Off-the signal descriptors. The values of the A's are then computeibeing equal to the slope of the C versus P curves at the nominal values of the Ps. Since it is the individual values of the param-' eters P; which are required, the equation C]=[A]P] can be solved. for P as follows:

where [A] or [B] is the inverse matrix of the matrix [A]. The inverse matrix [A]" is computed using standard matrix algebra techniques as the values ofA are known. The individual values of the parameters can now be determined by multiplying the matrix [B] and the column matrix [C]. This multiplication is performed by the polarity switches '42, the potentiometers 44 and the summing amplifiers 46. Thus, the polarities of the matrix elements'B are selected by the switches 42,-the amplitudes of the matrix elements B are set by adjusting the wiper arms of the potentiomcters 44, and the summing amplifiers give outputs proportional to the existing test specimen parameter changes on which the calibration is ased.-

Reference is now made to FIGURE 3 for a more detailed explanation of an operational test signal generating apparatus and signal expander for the device of FIG- URE 2. The apparatus of FIGURE 3 was used for analyzing resistivity changes in each layer of afour-layer sample. It is to be understood that the present invention is not to be limited to this number and type of parameter variable. Other parameter variables may be analyzed using the present invention, for example, the location, depth and presence of subsurface flaws and cladding thickness.

A crystal oscillator generates a 100 kc. frequency which is transmitted to the 100 kc. generator section 52 of a commercial impedance comparator 54 (for pIitude-phas: detector section. The oscillators in the generator sections 52 and 60 of the comparators 54 and 62' arenot stable enough for the present device, hence,

the 100. kc. crystal oscillator is used toachieve' stability.

The frequency divider 58 comprises the usual input amplifier, a decade counter, and a bandpass filter. The input amplifier of the divider 58 drives the decade counter to give positive triggering and divide the frequency by ten. The bandpass filter thereof smoothes the 10 kc. output signal therefrom.

'The kc. crystal oscillator 50 as stated stabilizes the frequency of the generator sections 52 and 60 of the impedance comparators 54 and 62 and also results in synchronization of the two generator sections. The synchronization is important because without it, continual shifting of the 100 kc. signal with respect to the 10 kc. signal would occurthroughout' the system and undesirable difference frequencies could interfere with measurements.

The generator section of each of the impedance comparators 54 and 62 furnishes the phase detector reference voltage internally. Each of the impedance comparators 54 and 62 has two amplitude-phase detector circuits, one

for indicating the amount of the signal in phase with afixed reference signal, and one for indicating the amount of signal in quadrature with the fixed reference. The reference signal must have the same frequency as the sigrml being analyzed and it must be derived from or synchronized with, the original generator source of the test coil driving signal.

The D.-C. outputs C Q C and 0, of the amplitude-phase detector sections 76 and 78 are thus proportional to the in-phase and quadrature components of the signal. The outputs C C C and C are not sinusoidal components themselves, but are direct currents proportional to these components. Thus, the outputs of a series of such amplitude-phase detectors represent the w...- .ww

. 7 It is to be understood that the signal descriptors used need not be limited to the Fourier coefiicients of the ex panded signal, but may consist of other characteristics of the'signal suchas amplitudes of the signal sampled at different times, various derivatives of the signal measured at different instants, successive derivatives of the signal at the same instant, various integrals of the signal or various combinations of these characteristics. Reference i is made to ficrosecond Sampler Handles 126 Channets, by M. T. Nadir, Electronics, January 23, 1959,

page 3 6,' and Electronic Analogue Computers, by G. A. Korn and T. M. Korn, 2nd Ed., 1956, McGraw-Hill Book Co., Inc., for circuits capable of obtaining these characteristics. A necessary requirement is that sufficient independence exists in the relationships between the parameters and the descriptors to make the desired sepis designed on the basis of an approximate linear relationship existing between parameter variations and the corresponding changes in test coil output f als. Where a nonlinear relationship occurs it has to be compensated for. For example, considerable compensation can be achieved by substituting nonlinear amplifiers for the linear amplifiers 38 shown in FIGURE 2. The characteristics of such amplifiers would be adjusted to compensate for the nonlinear variation of values of the outputs C C C34 and C33. a

A multidimensional excitation current is-herein defined to be a current represented by a superpositionof more than two basis functions having prescribed amplitudes. Each basis function corresponds to a dimension of the excitation current. The basis functions are the characteristic functions of ageneralized Fourier series expansion of the excitation. current two dimensions. In the two-frequency case hereinbefore described, the four. basis functions arethe sine and cosine- (or quadrature) components of the 10 kc. and 100 kc. excitation current. The amplitudes of the basis functions then represent the components of the foundimensional current projected on four mutually orthogonal signal space axes.

In FIGURE 4, an alternate analyzer section is shown for the device of FIGURE 1. For the analyzer of FIG- URE 4 the test coils and balance coils are driven with a broadband current function and the output of the coils is analyzed by expanding it intoan N-dimensional sig' nal space along specially selected coordinate axes.

The output signal from amplifier 18 is fed to a time reversal device 80 which serves to reverse the signal in time. This results in changing the signal from one which decays as time increases to one which grows as time increases since the latter is more easily operated on by the For thepurposes of this definition the word multidimensiond means more than orthonormal filter 82. The construction of the time re-.

The output of the time reversal device is fed to an orthonormal exponential filter 82 which expands the signal in terms of selected orthogonalized exponential components, an'l' there appears at each terminal a running signal whose value at each instant represents the coelfi cient of that particular orthogonal component representing the signal for times prior to that instant. I

' The outputs from the filter 82 are fed to a time domain sampler 84 which samples each output from the orthonormal filter 82 at a selected time. The time of sampling is determined by a time pulse transmitted from generator 10 through a time delay circu t 86 to the time domain sampler 84. The time delay circuit 86 has an adjustable time delay and comprises a driven multivibra'tcr, an adiustable phantastron pulse delay, .and. a

blocking oscillator. I

The outpu's of the time domain 'sampler 84 are a son ries, of voltages wh ch :emain constant as long as the i I test specimen parameters remain fixed, but which vary individually when the parameters change. These voltages represent the response vector in signal space on a .basis dependent'on the design of the orthonormal filters The outputs of the time domain sampler 84 are then fed to a signal space analyzer 88 which operates on the response vector to read out the various test specimen parameters individually. Generally, the operation of the space'analyzer 88 depends on the design of the filter 82 and the selected driving function applied to the test coils 12. Thus the analyzer 88 may be of several forms. The

. first is where the orthonormal filter-'82 together with the test coil drive functions are selected so as to make the various outputs from the filter 82 indicate the desired parameters independently. The second is where the analyzer 88 operates on the transmitted response function components to recombine them inlinearand nonlinear circuits; The output of the sampler. is determined experimentally for given variations in the parameters of the test specimen and the analyzer 88 is adjusted empirically to give an optimum independence of output readings. Third, the analyzer 88- is based on the hypothesis that the variation of each parameter of the test specimen generates by itself a geometrical object in N-dimensional signal space which can be identified with and calibrated in terms of that parameter irrespective of the values of the other parameters. Here, the parameter 88 gives a meassure of the geometrical entities for each parameter and compares them with a precalibrated' basis, performing the necessary interpolation and reading out the pararu-- eters independently.

Persons skilled in the art will, of course, readily adapt the general teachings of the invention to embodiments far different than the embodiments illustrated. Accordingly, the, scope of theprotection afforded the invention should'not be limited to the particularembodiments illustrated in the drawings and described above, but should be determined only in accordance with the append claims.

What is claimed is:

1. An eddy'current device for measuring multiple paof said coil into signals proportional to the coefiicicnts of the Fourier expansion of said received signal, and means for combining said expanded signals to give a plurality of outputs each responsive to a variable parameter of said metal sample.

2. An eddy current device for measuring multiple parameter variables of a-metal sample comprising a'test coil, means for applying a multidimensional excitation current to said test coil, means for null balancing the received signal of said test coil -for predetermined values of the variable parameters of said metal sample, means for expanding variations in received signal of said test coil from said null balance into signals proportional to the coefficients of the Fourier expansion of said variations in received signal, said expanded signals being linearly proportional to the variable parameters of said metal sample, and means for combining said expanded signals to give a plurality of outputs each responsive to avariable parameter of said metal sample.

3. The device according to claim 2 wherein said null balancing meanscomprise a reference coil identical to 7 said test coil and excited by the same multidimensional current as said test coil, a reference metal sample having parameters of known value, and a balance trimmer 4. The device accordingto claim 3 wherein said com-' bining means comprise means for multiplying a matrix [B] and a column matrix C] for the solution of the equation P]=[B] C], P] being equal to the parameter values of said metal sample arranged in a column matrix,

[B] being equal to the. inverse matrix of the matrix relating the P to C and C] being equal to said expanded signals arranged in a column matrix, and means for adding the multiplied matrix elements C, B in accordance with the solution of the equation P]=[B] C].

5. The device according to claim 4 wherein said matrix multiplying means comprise means for amplifying said expanded signals, means for generating opposing polarities for each of said expanded signals, means for selecting the polarity of each of said expandedsignals in accordance with the polari ies of the B elements of the inverse matrix relating the P to C; for'the solution of said parameter values of said metal sample arranged in said column matrix, means for multiplying each of said expanded signals of selected polarity with the amplitudes of the B elements of the inverse matrix relating the P, to Q for the solution of said parameter values of said metal sample arranged in said column matrix, whereby the B elements of the inverse matrix are multiplied by said expanded signals arranged in a column matrix. 6. The device according to claim 5 wherein said multiplying means comprise a plurality of potentiometers, said expanded signal being applied across the terminals of said potentiometers and the wiper arms of said potentiometers being set at predetermined values proportional to the values of the B elements associated with the corresponding expanded signals, whereby the output of the wiper arms of said potentiomcterserepresents the multiplication of the B elements of the inverse matrix and the column matrix of the expandedsignals. I

7. Art eddy current'device for measuring multiple parameter variables comprising a test coil, means for applying a multifrequency excitation signal to said test coil, means for filtering the received signalof said coil into the individual frequencies thereof, means for generating signals proportional to the amount of said frequencies in phase and in quadrature with like frequency reference signals derived from the excitation signal, said generated signals being proportional to the coeflicients in a Fourier series expansion of the received signal, and means forcornb'ming said generated signals to give a plurality of outputs each responsive to a variable param" eter of said metal sample.

8. An eddy current device for measuring four parameter variables of a metal sample comprising means for generating a first signal at a predetermined frequency,

means for generating a second signal at a frequency different than said first signal but synchronized therewith, 21 test coil, a reference coil identical to said test coil, a

reference sample having four parameters of known value, means for applying said first and second signals to said test and reference coils, means for balancing the received signals of said test and reference coils to give a null output for parameter values of said metal sample being' equal to parameter values of said reference sample, a first filter having a selective bandpass at the. frequency of said first signal, a second filter having a bandpass 'at the frequency of said second signal, the input of said first and second filters being connected to the output of'said null balance means, a first amplitude-phase detector having'its input connected to the output of said first filter, a second amplitude-phase detector having its input connected tothe output of said second filter, means for providing a reference signal to said first and second amplitude-phase detectors'from said first and second signal generating means respectively, the outputs of said first and second amplitude-phase detectors being proportional to the amount of the outputs of said first and second filters References Cited by the UNITED STATES PATENTS 2,744,233 5/ 1956 Paivinen 324-34 2,394,203 7/1959 Cory 324- 2,950,435 8/1960 LOCher '-.t. -e-.. 324-77 2,955,840 12/ 1960 Renken et'al. 324-40 2,970,469 2/1961 Feldman 324-77 3,015,949 1/1962 Arnold 73'67.2